CEOSEA '98 Pl'Oceeoil1.1J, Ceo !. Soc . IHa/aYJia BIIII. 45, Decemher /999; pp. 557-545 Ninth Regional Congress on Geology, Mineral and GEOSEA '98 Energy Resources of Southeast Asia - GEOSEA '98 17 - 19 August 1998 • Shangri-La Hotel, Kuala Lumpur, Malaysia Magnetic and gravity fields in southern Zambales: implications on the evolution of the Zambales Ophiolite Complex, Luzon, Philippines l CARLA B. DIMALANTAl ,2, GRACIANO P. YUMUL JR.!, JOEL V. DE JESUS AND DECIBEL V. F AUSTINOl 'Rushurgent Working Group, National Institute of Geological Sciences College of Science, University of the Philippines Diliman, Quezon City, Philippines 20cean Research Institute, University of Tokyo 1-15-1 Minamidai, Nakano-ku Tokyo 164-8639, Japan Abstract: Ophiolites are rock associations representing crust-mantle sequences. Interest in these rock suites is reflected in the extensive geological, geochemical and petrological studies that have been carried out. It is only recently, however, that geophysical tools have found application in the study of these sequences. In the Philippines, the southern Zambales area was the subject of one of the few geophysical investigations applied on ophiolite complexes. The magnetic and gravity surveys conducted in the southern portion ofthe Zambales Ophiolite Complex defined a northwest-southeast linear feature which is believed to correspond to the Subic Bay Fault Zone. The fault zone was delineated on the basis of the steep gradients displayed by the magnetic anomalies and from inflections noted in the gravity contours. This structure is believed to be the tectonic contact between the central (Cabangan) and southern (San Antonio) massifs of this ophiolite complex. Recognition of the fault zone is consistent with a model which suggests that the San Antonio massif was a rifted unit from the Acoje block of the Masinloc Massif. The translation southward of this massif may have occurred along the Subic Bay Fault Zone. INTRODUCTION investigations. Extensive geological surveys (e.g. Hawkins and Evans, 1983; Bacuta, 1989; Rossman Ophiolites are suites of rocks that are believed et al. , 1989) and geochemical studies (e.g. Gearyet to correspond to crust-upper mantle sequences. al., 1989; Evans et al., 1991; Hawkins and Florendo, Initially, interest in these rock sequences was 1992; Yumul, 1992, 1996) have been carried out. due to the presence of a variety of mineral deposits. However, some questions remained unanswered Subsequently, however, the focus of various regarding the evolution of this ophiolite complex. scientific investigations shifted to obtaining The present-day scenario shows the ophiolite information on how these sequences evolved and complex being divided into several units with how they were emplaced. Recent developments different geochemical characteristics (i.e. the Acoje have seen the increasing use of geophysical block and San Antonio massif manifest island arc investigations, specifically magnetic and gravity signatures while the Coto block and the Cabangan surveys, in the study ofthese ophiolite complexes massif show transitional mid-ocean ridge basalt - (e.g. Manghnani and Coleman, 1981; Milsom, island arc charact eristics). One question that 1991; Milsom et al., 1996; Ravaut et al. , 1997). In needed to be addressed was how can an island arc the Philippines, however, there have been limited terrane (San Antonio massif) be juxtaposed to a applications of geophysical techniques in the study mid-ocean ridge crust (Cabangan massif)? of ophiolites and ophiolitic complexes (e.g. Recent geological and geochemical data suggests Dimalanta, 1996, Dimalanta et al., 1998; Barretto the presence of a major structural feature (e.g. et al., 1998). Yumul et al., 1990; Yumul and Dimalanta, 1997a) The complete ophiolite suite in the Zambales which helps elucidate the observed arc terrane area has been the subject of numerous geoscientific accretion. Given the possibility that a major 538 CARLA B. DIMALANTA, GRACIANO P. YUMUL JR., JOEL V. DE JESUS AND DECIBEL V. FAUSTINO structural feature may be present in the southern from the Acoje block of the Masinloc massif. The part of the Zambales region warranted the conduct presence of a major tectonic feature was involved of geophysical surveys in the area. The role of this along which this translation took place. structural feature in the evolution of the Zambales In order to establish the presence of this Ophiolite Complex has been discussed extensively structural feature which is deemed to have played elsewhere (e.g. Yumul and Dimalanta, 1997a) but a significant role in the evolution of this ophiolite will be reiterated in the subsequent discussion. complex, combined ground and magnetic surveys were conducted in the southern part of ZOC. The GEOLOGICAL BACKGROUND area covered by the magnetic and gravity surveys is located between 14°50' and 15°10'N and 120°00' The Zambales Ophiolite Complex (ZOC) in and 120020'E (Fig. 1). Luzon, Philippines is a supra-subduction zone ophiolite formed in a marginal basin (e.g. Geary et MAGNETICS al., 1989; Yumul and Dimalanta, 1997a). It underlies the Zambales area in the western part of The gravity and magnetic methods of Luzon Island, Philippines. This north-south geophysical prospecting are effective in delineating trending, east-dipping ophiolite suite consists of faults and similar structural features (e.g. Wright, residual harzburgites and lherzolites, transition 1981; Irvine and Smith, 1990; Zeng et al., 1997). zone dunites, layered ultramafic and mafic Combining the magnetic investigation with a cumulates, dike-sill complexes and volcanic rocks gravity survey would, hence, serve to confirm or (Fig. 1). verifY the presence of any structural feature. Three massifs - Masinloc, Cabangan and San The total magnetic intensity anomaly map for Antonio - comprise the ZOC (Fig. 1). Due to the the study area was generated using measurements observed petrological and geochemical differences, obtained from three hundred and thirty stations. the Masinloc massif is further divided into two Details of data acquisition and reduction are units - the Acoje and Coto blocks. Moreover, presented elsewhere (Yumul et al., 1998b). available data show that the Cabangan massif and Total magnetic field intensities in the southern the Coto block are geochemically similar portion of ZOC range from 40800 to 41300 nT. (transitional mid ocean ridge-island arc) while the Relatively low magnetic anomalies are noted in San Antonio massif exhibits the same island arc areas underlain by sediments such as the Castillejos (IA) characteristics as the Acoje block. The present­ area and southeast of the Sindol Hill area. On the day configuration of this ophiolite complex proved other hand, magnetic highs are observed in the to be intriguing to those who were studying the Subic and Sindol Hill areas. Pillow basalts area. (magnetic susceptibility 25 SI units) intruded by In addition, there was the question of the sheeted diabase dikes (magnetic susceptibility 0.18- juxtaposition of an island arc terrane (San Antonio 0.46 SI units) are encountered in the Subic-Olongapo massif) to a mid-ocean ridge/marginal basin-like area. These rock units display a large magnetic crust (Cabangan massif),_ as seen fro:rp. an east-west susceptibility contrast which produced the high section in the southernmost portion of the ZOC anomaly. The magnetic high in the Sindol Hill (Yumul et al., 1990). Another enigma is presented area is attributed to the presence of highly by the small hills noted along the western edge of serpentinized clinopyroxenites. Magnetic the Cabangan massif (Yumul and Dimalanta, susceptibility measurement of a sample from Sindol 1997a) (Fig. 1). Closer investigation of these small Hill gave a value of 13.08 SI units. This is consistent hills reveal that they consist of layered with observations noted elsewhere regarding the clinopyroxenites and gabbronorites. Available high magnetic susceptibilities offully serpentinized geologic information clearly shows that the ultramafic rocks (Clark et al., 1992). Cabangan massif does not have a well-developed The magnetic intensities gradually decrease layered clinopyroxenite-gabbronorite member from Subic to Cawag. The Cawag section is within its ultramafic-mafic sequence. But such underlain by gabbronorites (27.45 SI units) on the layered clinopyroxenites and gabbronorites are east. These are in fault contact with intensely reported to be extensive in the Acoje block and San sheared, highly weathered clinopyroxenites (0.10 Antonio massif. If these small hills are not part of SI units) on the west (Yumul and Dimalanta, 1997b). Cabangan massif, then how can their presence along A relatively pronounced closed contour high can be the western edge ofCabangan massif be explained? noted northwest of the Castillejos area. This Yumul et al. (1998a; 1998b) suggest that these anomaly high might conceivably owe its presence small hills were left behind as the San Antonio to the dacite plugs in that vicinity. Magnetic massif was translated southward after being rifted susceptibility measurements of dacite samples gave GEOSEA '98 Proceeuing" (GSM BuLL. 45) MAGNETIC AND GRAVITY FIELDS IN SOUTHERN ZAMBALES 539 values of 2.39 and 3.94 81 units. produced utilizing one hundred and ninety one Another significant feature of the magnetic measurements taken with a LaCoste and Romberg anomaly map is the steep gradient that separates gravity meter. Details on data acquisition and the western side from the eastern side. Relatively reduction are presented elsewhere (Yumul et al., steep, closely spaced contours characterize the 1998b). eastern side while broadly spaced contours can be The gravity anomaly map is characterized by noted on the west CYumul et al., 1998b) (Figs. 2 and 3). the gradual increase of anomalies (from 220 to 270 mgals) from the central to the northwestern part of GRAVITY the study area (Figs. 2 and 3). The main geological factors causing the gravity anomalies are the In order to verify the results obtained from the different units ofthe ophiolite complex. The central magnetic survey, a follow-up ground gravity survey portion of the area is characterized by low gravity was conducted. The gravity anomaly map was anomalies.